Ion lasers are relatively reliable sources of continuous wave green laser light with low amplitude noise and provide output power at the multiple watt level, but these devices convert electrical power into optical power with efficiencies of only a small fraction of one percent. There are many applications that would benefit from the development of a highly efficient, low cost, diode pumped, continuous wave, solid state green laser source, also at the multiple watt level and with comparable amplitude stability.
Certain fundamental difficulties with intracavity frequency doubled lasers were discovered and numerically modeled in early work by Baer. See for example T. Baer, J.Opt. Soc. Am. B., Vol. 3, No. 9, pp. 1175-1180 (1986), and U.S. Pat. Nos. 4,656,635 and 4,701,929. It was reported and disclosed that large amplitude fluctuations are observed on the green output beam and the intracavity laser beam when a frequency doubling crystal like KTP is introduced into an otherwise amplitude stable multiaxial mode diode pumped Nd:YAG laser. It was also reported that the large amplitude noise on the green output beam disappears when an appropriate etalon is placed in the laser cavity that forces single axial mode oscillation. In the multiaxial mode case, where 2 to 4 modes were oscillating, the green output power was seen to fluctuate with up to 100% modulation depth. Baer's experimental work and theoretical model indicated that the insertion of a frequency doubling crystal in this multiaxial mode laser resulted in nonlinear coupling of the loss of the axial modes via sum frequency generation. A high peak power in one axial mode induced a high nonlinear loss for the other axial modes, and caused an unexpected and undesirable pulsing effect.
As an example of the effect described by Baer, a laser with two infrared axial modes generated three green frequencies; two were doubled modes and the other a sum frequency mode. The sum frequency process couples the two infrared axial modes in a way that can cause them to switch on and off in a sequential fashion. The typical period of this mode coupling was found to be a function of the magnitude of the nonlinear conversion. For weak conversion, the period was short and the modes minimally modulated. For stronger conversion, the mode coupling period lengthened, and the modes switched on and off in pulses of high peak power, completely out of phase with each other. The noise spectrum of such a laser typically showed substantial peaks in the 10 to hundreds of kilohertz range for either the green or infrared, and corresponded to considerable amplitude fluctuations.
A source with this type of amplitude modulation is not as generally useful as one with low amplitude noise. As an example, for applications in opthamology, amplitude stability is required on the time scale of the typical exposure durations for accurate control of therapeutic effects. Another example is the use of a green laser as a pump for a second laser, such as a dye or Ti:Al.sub.2 O.sub.3 laser. Deep amplitude modulation at certain frequencies can cause undesirable amplitude modulations on the output of the second laser.
A number of methods for stabilizing the intracavity frequency doubled output of a diode pumped solid state laser have been described and demonstrated. The most common materials have been Nd:YAG as a laser medium and KTP as a nonlinear (doubling) medium. For this reason, the most common type of phase matching is Type II. Techniques that have been used in attempt to stabilize the frequency doubled output from such systems have included insertion of intracavity quarter wave plates (see M. Oka, and S. Kubota, Opt. Lett. 13, 805 (1988)), optical cavity temperature control (see U.S. Pat. No. 4,884,277 issued to Anthon et al. on Nov. 28, 1989) and forcing single frequency operation (see U.S. Pat. No. 5,164,947 issued to G. J. Lukas et al. on Nov. 17, 1992). While all of these techniques have demonstrated regimes of operation where the frequency doubled output is measured to have low amplitude noise, in all cases the techniques are difficult to implement in a reliable, low cost fashion that is resilient to changes in environmental conditions, such as temperature. The techniques employed typically must maintain an inherently amplitude-unstable system within the narrow range of parameter space for which the system is stable.
It would be highly desirable to provide an amplitude stable, intracavity frequency doubled laser that does not require active stabilization or single axial mode operation. Additionally, there is a need for a laser of this type that remains stable over a range of environmental conditions, such as changes in ambient temperature.